1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus capable of compensating for the deterioration in efficiency of the horizontal transfer of signal charge visually conspicuous in an image output in, e.g. a high ISO (International Standards Organization) sensitivity mode.
2. Description of the Background Art
With a conventional solid-state image pickup apparatus of the type operable in a high ISO sensitivity mode, a problem is that a signal level output from an image sensor after picking up an image of a subject field is decreasing with an increase in the number of pixels, or photosensitive cells, of the imaging frame and a decrease in the size of the individual pixel. Another problem is that during horizontal transfer of signal charge in the image sensor a signal amount received from the preceding pixel and a charge amount left in the following pixel have an upwardly convex characteristic with respect to the signal level, so that the ratio of a transfer remainder increases with respect to the original signal in the form of an offset and critically degrades colors of the captured image. This is particularly true when the amount of signal charges to be transferred is small.
On the other hand, in the high ISO sensitivity mode, a type of image pickup apparatus executes gain control on an image signal output from the image sensor with its high gain. At this instant, because noise is conspicuous in the resulting image due to the deterioration in efficiency of the horizontal transfer of signal charge, it is necessary to accurately measure the horizontal transfer efficiency and examine the image sensor on the basis of the result of measurement.
In light of the above, Japanese patent laid-open publication No. 2004-327722, for example, discloses a solid-state image sensor configured such that at the manufacturing process image data A and B are produced from two kinds of photodiodes each storing a particular amount of signal charge. Subsequently, mean values A1 and B1 are calculated of differences between signals output from two nearby pixels around an electric-charge concentrating portion by using the image data A and B. Also, other mean values A2 and B2 are calculated of differences of signals output from two nearby pixels at a portion remote from the concentrating portion by also using the image data A and B. A ratio T=A1/B1 and a ratio V=A1/B1 are used to determine whether or not a defect of the concentrating portion is acceptable, and then the ratio T and a ratio U=A2/B2 are used to determine whether or not a defect of the horizontal transfer path is acceptable.
To compensate for the deterioration of horizontal transfer efficiency particular to a solid-state image pickup apparatus, correction values for signal processing may be calculated beforehand. However, differences between specific products of image sensor cannot be reflected by the correction unless the characteristic of the individual image sensor is measured beforehand. Further, the characteristic of the horizontal transfer efficiency of an image sensor is apt to vary in dependence upon ambient temperature and other environmental conditions. For example, when charges are transferred by drift, the characteristic of the horizontal transfer efficiency is considered to deteriorate in a low-temperature environment.
However, the solid-state image sensor taught in laid-open publication No. 2004-327722 mentioned earlier is assumed to use an image sensor originally having defects at least in its horizontal transfer portion, and is therefore unable to compensate for the deterioration of horizontal transfer efficiency in the event of image pickup. Moreover, the image pickup apparatus disclosed in the above document has its basis on a manufacturing process making correction on the horizontal transfer efficiency, so that it is extremely difficult to accomplish horizontal-transfer efficiency correction dependent on specific products of image sensor.